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The hematocrit (Hct) is 68% in a newborn. Polycythemia is an increase in total erythrocyte mass empirically defined as a venous Hct >2 standard deviations above the normal value for postnatal and gestational age. An infant who has a peripheral venous Hct >65% or a hemoglobin >22 g/dL is considered polycythemic. Polycythemia occurs in 1% to 5% of healthy newborns (1%–2% at sea level and 5% at high altitude) and is rare in premature infants <34 weeks’ gestation. The venous Hct level of 65% was chosen to define polycythemia based on an exponential increase in viscosity above this level. Hyperviscosity and the resultant impairment of blood flow are important in polycythemia because they cause most of the symptoms and complications. Increased blood viscosity impairs circulation of red blood cells through vessels and organs and interferes with delivery of oxygen, glucose, and amino acids to the tissues. Poor oxygen and nutrient delivery causes neurologic, gastrointestinal, metabolic, and cardiopulmonary problems. The Hct is the main determinant of blood viscosity (thickness); however, plasma proteins (especially fibrinogen), low plasma volume, and decreased erythrocyte deformability also affect blood viscosity. Blood is unable to flow freely due to increased viscosity of blood because of the increased number of erythrocytes that occurs in polycythemia. Hyperviscosity is defined as >14.6 centipoise (shear rate of 11.5 seconds) and is also defined as 2 standard deviations greater than the norm. Viscosity rises linearly until the Hct reaches 60% but increases exponentially when it reaches 70%.

Polycythemia hyperviscosity syndrome (PHS) is the symptom complex that involves hyperviscosity resulting from polycythemia and the symptoms that accompany it. Not all polycythemic infants will have hyperviscosity: roughly 47% of polycythemic infants will have hyperviscosity, and 24% of infants with hyperviscosity have polycythemia.


  1. What is the central hematocrit? If the Hct was obtained by a heel stick, it can be falsely elevated by up to 15% as compared to venous samples. Warming the heel prior to the heel stick will give a result that better correlates with the central Hct. However, treatment should never be initiated based on heel stick Hct values alone; a central (peripheral venous phlebotomy) Hct is needed. The Hct is highest in capillary samples and lowest in umbilical venous samples.

  2. What method was used to determine the hematocrit? There are two methods of determining the Hct: automated hematology analyzer (cell counter), which calculates the Hct from a measurement of the mean cell volume and hemoglobin, and the microcentrifuge (blood is centrifuged, plasma and packed cell volume separates, and the Hct is measured). A microcentrifuge Hct can be 2% higher than Hct by a hematology analyzer.

  3. How old is the infant? A newborn’s Hct peaks at 2 hours of age, plateaus between 2 and 4 hours of age, and then decreases back to values close to cord blood values between 12 and 24 hours of age. ...

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